This invention relates to a carrier regenerating device for use in a mobile station of a mobile satellite communication network.
In mobile satellite communication, a modulated signal is transmitted from a master station to mobile stations and from each mobile station to the master station. The modulated signal is derived by modulating a carrier signal at a frame period by a data signal representative of a message and by unique words which are periodically interspersed in the data signal. Therefore, the modulated signal comprises a modulated data signal and modulated unique words and will herein be called a modulated overall signal. The unique words are preliminarily known at the mobile stations. Such a unique word will herein be called a locally known unique word.
In order to get primarily the message, each mobile station comprises a carrier regenerating device for regenerating the carrier signal as a regenerated carrier signal from the modulated overall signal received at the mobile station as an input modulated signal having the frame period and comprising the modulated data signal and the modulated unique words which are periodically interspersed in the modulated data signal.
A conventional carrier regenerating device comprises a demodulating circuit for quadrature demodulating the input modulated signal into a demodulated overall signal by the use of the regenerated carrier signal. The demodulated overall signal comprises a demodulated data signal representative of the message and demodulated unique words. The conventional regenerating device further comprises a carrier reproducing or regenerating circuit for regenerating the regenerating carrier signal from the demodulated overall signal.
The conventional carrier regenerating device operates well as 10 dg as the input modulated signal supplied thereto is steadily above a predetermined signal to noise (S/N) ratio. However, the carrier regenerating device may not always be supplied with the input modulated signal above the predetermined signal to noise ratio. For example, as the mobile station is moved from ode location to another it may fall within the shadowed buildings and trees depending on the location. In a worst case, the carrier regenerating device can not receive the input modulated signal at all. In such an event, the carrier regenerating circuit is placed in a free running state. The regenerated carrier signal steps out of frame synchronism, namely, out of synchronism with the frame period. As a result, a phase shift occurs in the regenerated carrier signal. In addition, a frequency shift appears in the regenerated carrier signal. Even when the mobile station moves to a location where the carrier regenerating device can receive the input modulated signal above the predetermined signal to noise ratio, it takes a long recovery time for the carrier regenerating circuit to regenerate the regenerated carrier signal again in synchronism with the frame period.
In order to regenerate the regenerated carrier signal in a short recovery time, an improved carrier regenerating device is disclosed in the European Patent No. 0427283A4. The improved carrier regenerating device comprises the above-mentioned demodulating and the carrier regenerating circuits. The improved carrier regenerating device further comprises a frame synchronizing circuit for producing an aperture signal by the use of the demodulated overall signal when the demodulated overall signal has a level which is lower than a predetermined threshold level. The aperture signal defines aperture intervals determined on the basis of the frame period. Responsive to the aperture signal, a cross correlating circuit calculates a cross-correlation coefficient between the demodulated overall signal and the locally known unique word. The phase shift is detected as a phase error in accordance with the cross-correlation coefficient in a phase error detector. In addition, the frequency shift is calculated as a frequency error in accordance with the phase error in a frequency error calculator.
When the frequency shift is larger than a predetermined shift value on receiving the input modulated signal again at the carrier regenerating device, the frequency error calculator can not calculate the frequency error. As a result, it is impossible for the carrier regenerating device to regenerate the regenerated carrier signal again in synchronism with the frame period.